Investigation of varying-composition gas injection for coalbed methane recovery enhancement: A simulation-based study

In this study, a varying-composition injection is proposed, as an alternative technique to the constant-composition injection. In a varying-composition injection, the composition of the injected gas is altered during the injection period through several steps. Such a scenario can postpone the breakthrough time, and meanwhile avoid the deterioration of well injectivity. To assess the proposed method and find an optimal and practical injection schedule, a semi-synthetic simulation model is constructed. Different injection scenarios are compared with each other, using a compositional simulator (ECLIPSE-300), which uses the extended Langmuir isotherm and the modified Palmer-Mansoori model. By carrying out a series of sensitivity analyses, an optimum scenario is found. The best obtained scenario is the one that begins by injecting a mixture with less CO2, and continues by a sequential rise in the CO2 fraction. The outcomes confirm that the proposed technique has the following benefits, in comparison with the optimum scenario of constant-composition injection: 1- greater Methane recovery, 2- higher Methane production rate 3- deferment in permeability reduction, 4- later N2 breakthrough.